iReboot is a sotware that allows you to choose which Operating System you want to boot your computer in.
It is very easy to use. You don't need to reboot the system in order to switch to another Operating System, the software shows in the taskbar at startup and lets you choose which Operating system to use.
iReboot is very light, only 253Kb, so it will not consume much memory space.
It is completely free, you don't have to buy any license.
The software is only for Windows XP and Vista only.